The present invention relates to an ion guide, an ion mobility spectrometer or separator, a mass spectrometer, a method of guiding ions, a method of separating ions and a method of mass spectrometry. The preferred embodiment relates to a device for and method of separating ions according to differences in their ion mobility.
It is known to provide an ion guide wherein ions are confined radially by RF fields and wherein a gaseous media is provided to the ion guide. In such circumstances it is known to drive ions forwards along and through the ion guide. For example, it is known to provide an axial field as part of a collision cell forming part of a tandem mass spectrometer wherein fast transit times are desirable e.g. when performing Multiple Reaction Monitoring (“MRM”), parent ion scanning or neutral loss experiments using a triple quadrupole mass spectrometer. Similar devices may also be used to separate ions according to their ion mobility and hybrid ion mobility-mass spectrometer instruments are used for a variety of different applications.
U.S. Pat. No. 6,914,241 (Giles) describes how ions may be separated according to their ion mobility, by progressively applying transient DC voltages along the length of an RF ion guide or ion mobility separator comprising a plurality of electrodes. The ion mobility separator may comprise an AC or RF ion guide such as a multipole rod set or a stacked ring set. The ion guide is segmented in the axial direction so that independent transient DC potentials may be applied to each segment. The transient DC potentials are superimposed on top of an AC or RF voltage (which acts to confine ions radially) and/or any constant DC offset voltage. The transient DC potentials generate a travelling wave which moves along the length of the ion guide in the axial direction and which acts to translate ions along the length of the ion mobility separator.
Another known ion mobility separation device comprises a drift tube comprising a series of rings wherein a constant potential difference is maintained between adjacent members such that a constant electric field is produced. A pulse of ions is introduced into the drift tube which contains a buffer gas and ions separate along the longitudinal axis according to their ion mobility. The device is operable at atmospheric pressure without RF confinement and can offer a resolution of up to 150 (Wu et, al. Anal. Chem, 1998, 70 4929-4938). Operation at lower pressures more suitable for hybrid ion mobility-mass spectrometer instruments leads to greater diffusion losses and lower resolution.
An RF pseudo-potential well may be arranged to confine ions radially and may be used to transportions efficiently by acting as an ion guide thereby solving the problem of diffusion losses. Ions may be propelled along the guide and ions may be separated according to their ion mobility. However, in order to achieve a high resolution of mobility separation at relatively low pressures, a relatively long drift tube must be employed in order to keep within the low field limit as described in more detail below.
In order to separate ions according to their mobility in an RF ion guide, an axial DC electric field may be generated which is orthogonal to the RF radial confinement. If a constant axial electric field E is applied in order to drive ions along and through an ion guide containing a gas, then the ion will acquire a characteristic velocity:νd=E·K  (1)wherein K is the ion mobility.
To achieve a mobility separation whereby ions acquire negligible energy compared to the background thermal energy of a gas, it is necessary to consider the parameter E/P, wherein P is the pressure of the neutral gas.
To maintain ion mobility separation in the so called low field regime whereby ions do not receive kinetic energy from the driving field it is necessary that the parameter E/P is maintained at a value less than about 2V/cm-mbar.
Under low field conditions in a drift tube having a length L and wherein a voltage drop V is applied the resolution is found to be independent of ion mobility and only dependent upon the voltage drop such that in the absence of space charge effects:
                              L                                                x              _                                                  =                              V                    0.173                                    (        2        )            wherein | x| is the mean displacement of the centre of mass of the moving ion cloud.
The parameter L/| x| is effectively the resolution of the mobility separation. It will therefore be apparent that the performance of the ion mobility spectrometer can be increased by applying voltage drops across the length of the drift tube.
In a hybrid ion mobility-mass spectrometer the typical pressure of the ion mobility drift region is in the region 0.5-1 mbar. Operating at pressures much greater than this puts great demands upon the vacuum system which needs to be differentially pumped in order for the mass spectrometer stages to operate efficiently.
At a typical drift tube length of 20 cm and an operating pressure of 0.5 mbar the maximum voltage that can be applied within the low field limit is 20 V. This results in a maximum resolution. of 26. In order to achieve a resolution of 100 under the same conditions would require a drift tube length having a length more than 3 meters long. However, this is impractical for commercial mass spectrometers.
It is therefore desired to provide an improved ion guide and ion mobility spectrometer or separator.